Moderate solvation structures of lithium ions for high-voltage lithium metal batteries at −40 °C

Abstract

Lithium metal batteries (LMBs) are considered highly promising due to their high-energy-density; however, they suffer from challenges such as lithium dendrite growth at low temperatures (LT) and severe decomposition at high cut-off voltages. Here, a quasi-solid-state electrolyte (QSSE) containing a carboxylic ester solvent with an ethoxy side difluoro-substitution group (–OCH₂CF₂H) has been developed. By withdrawing the electron cloud of the carbonyl group (CO) and transferring it to the fluorine atoms, the –OCH₂CF₂H group achieves a balanced charge dispersion between the fluorine and carbonyl oxygen atoms. Consequently, the QSSE forms a moderate solvation sheath through Li–F and Li–O coordination with the fluorinated carboxylic ester solvent, which not only promotes the de-solvation of Li⁺ at low temperatures but also induces the formation of a LiF-rich interphase to suppress lithium dendrite growth and detrimental side reactions on the cathodes’ surfaces. As a result, the QSSE enables stable cycling of a high-voltage Li‖LiNi_0.6Mn_0.2Co_0.2O₂ (NCM622) cell at 4.6 V, with a high-capacity retention of 85% and an average coulombic efficiency (CE) exceeding 99.9% over 700 cycles at −20 °C. Even at a lower temperature of −40 °C, the Li‖NCM622 cell provides a high capacity retention of 87.9% after 125 cycles. Moreover, a prototype 450 W h kg⁻¹ pouch cell (2.9 A h) operates for 75 cycles at −20 °C with 83.4% capacity retention using a low electrolyte/capacity (E/C) ratio of 1.5 g A h⁻¹. This design strategy provides a promising approach for future exploration of high-voltage lithium metal batteries under low-temperature conditions.